There has recently been an industry focus on digital streaming of content to mobile, portable, and handheld receivers through terrestrial broadcast systems. This type of broadcasting is being developed for implementation in licensed UHF frequency bands such as 0.7 GHz to 1.0 GHz (upper L-Band: TV channels 52 and above; mobile radio) and 1 GHz to 2 GHz (lower S-band).
At L-Band frequencies, the preferred method of transmission is vertical polarization. There are at present two styles of vertically polarized antennas that are readily available for commercial use in transmission at these microwave frequencies, namely panel and whip antennas. Panel antennas are intrinsically directional in nature and are typically used to cover sectors of space. Whip antennas are nominally omnidirectional and are used preferentially in applications requiring substantially equal radiation in all azimuths.
The shortcomings of a vertically polarized collinear dipole antenna include limited capacity to realize beam tilt. Increased input loading with additional dipoles constrains input transformer performance for both power and bandwidth. Structural support is provided largely by the radome.
The shortcomings of panel antennas include requirements to provide extensive systems of power dividers and feed lines where multiple panels must transmit carefully phased inputs, a panel or an array of panels pointing in each direction (typically four quadrants for omnidirectional capability, with overall antenna gain dependent on array size), use of a tower with multiple discrete units mounted thereon, and accommodation of wind loading from multiple units.
The vertically polarized traveling wave antenna apparatus, means, and design methods disclosed in the '644 application permit production of an omnidirectional antenna that permits simplicity in its mechanical construction, minimal design adaptation to vary beam tilt and null fill, matched input impedance substantially independent of the number of elements, excellent azimuth pattern circularity, and moderate power capability.
Some omnidirectional antennas are useful in many but not all applications. For example, in an open environment in a city, need for mobile broadcast service may surround a transmitter site, so that an omnidirectional antenna is appropriate. However, in other environments, such as along highways, it may be preferable to supply service only or primarily in line with the roadway, which can allow narrower focus of the same energy, permitting fewer or less power-consuming devices to achieve a level of coverage.
The known antennas for providing such patterns are largely limited to the above-referenced panel radiators and arrays thereof. Such panels are effectively unidirectional, with a single beam having breadth that depends on the intrinsic gain of the individual panel, increasingly narrow as the number of cofiring panels in an array increases. If a single site is intended for placement midway along a substantially straight section of road, for example, it is necessary to place two panels (or stacks of panels) back-to-back to provide a so-called “peanut” propagation pattern. This produces deep nulls to the sides, which are potentially unacceptable for mobile coverage, and may necessitate adding one or more auxiliary panels oriented in the short-range directions.
As the desired gain/range/beam narrowness of the transmitter site increases, and thus the number of panels, complexity increases. Each panel must be fed, so the original signal must be split using power dividers and feed lines. Each added connection has the potential to reduce system reliability. Feed for auxiliary panels must be provided at power levels suited to the desired azimuth pattern.
Panel antennas may also be more configurationally complex than traveling wave dipoles in some embodiments. Thus, there are significant limitations in some antenna types when considered for the power, economy, and coverage of broadcasting applications to which the invention is directed.